1. Technical Field of the Invention
The present invention relates generally to a method of fabricating a spark plug equipped with a piezoelectric sensor working to measure the pressure of combustion of fuel in internal combustion engines and a spark plug fabricated by the same.
2. Background Art
Japanese Utility Model First Publication No. 58-23194 discloses a spark plug with a piezoelectric sensor installed on the periphery of a porcelain insulator within a housing. An annular holder or nut is screwed into the housing to apply a preload to the piezoelectric sensor which is required for the piezoelectric sensor to transform mechanical deformation thereof into an electric voltage signal.
The above type of spark plug, however, encounters the drawback in that the nut screwed into the housing abuts directly on the piezoelectric sensor, thus resulting in distortion of the piezoelectric sensor. In the worst case, the breakage of the piezoelectric sensor results.
It is usually necessary to place the piezoelectric sensor in a waterproof environmental condition. It is, however, difficult to avoid entrance of water from between the nut and the housing completely.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide a fabrication method of spark plugs for avoiding breakage of a piezoelectric sensor subjected to a preload.
It is a further object of the invention to provide a fabrication method of spark plugs for avoiding entrance of water into a piezoelectric sensor.
It is a still further object of the invention to provide a spark plug made by either of the above fabrication methods.
According to one aspect of the invention, there is provided a spark plug fabricating method which comprises the steps of: (a) preparing a press jig; (b) preparing a spark plug assembly made up of a hollow cylindrical porcelain insulator having a given length in which a center electrode is disposed, a housing in which the center electrode is retained through the porcelain insulator and which has a ground electrode installed thereon, a piezoelectric sensor designed to be responsive to pressure of combustion of fuel within an engine transmitted through the porcelain insulator to provide a signal indicative thereof, and a holder put on the housing for holding the piezoelectric sensor mechanically; (c) placing the spark plug assembly within the press jig; (d) pressing the holder to apply a given preload to the piezoelectric sensor in a longitudinal direction of the porcelain insulator; and (e) joining the holder to the housing while maintaining the preload applied to the piezoelectric sensor to fabricate a spark plug completely.
In the preferred mode of the invention, the press jig includes a press block having opposed ends one of which abuts on the holder and a screw which abuts on the other end of the press block and is rotated to press the holder through the press block.
The holder may be jointed to the housing by at least one of staking and welding.
According to the second aspect of the invention, there is provided a spark plug which may be employed in a gas engine of a generator in cogeneration systems or automotive internal combustion engines. The spark plug comprises: (a) a center electrode; (b) a porcelain insulator having a first and a second length, the first length having the center electrode retained therein; (c) a hollow cylindrical housing in which the center electrode is retained through the porcelain insulator and which has a ground electrode installed thereon; (d) a piezoelectric sensor disposed on an outer periphery of the second length of the porcelain insulator, the piezoelectric sensor being responsive to pressure of combustion of fuel within an engine transmitted through the porcelain insulator to provide a signal indicative thereof; and (e) a holder welded to the housing so as to apply a given preload to the piezoelectric sensor at all times.
In the preferred mode of the invention, the holder is made of a hollow cylinder one end of which is welded to an overall circumference of the housing. A sealing member is disposed between the other end of the hollow cylinder and the porcelain insulator. The piezoelectric sensor is disposed within a chamber formed between the holder and the porcelain insulator.
The holder may also include an annular disc. The hollow cylinder is welded at the one end thereof to the overall circumference of the housing and at the other end to the whole of an outer circumference of the annular disc. The sealing member may be disposed between an inner periphery of the annular disc and the porcelain insulator. The piezoelectric sensor is disposed within a chamber defined by the hollow cylinder, the annular disc, and the porcelain insulator.
A chamber is defined within which the piezoelectric sensor is disposed and filled with a resinous material.
A shoulder may be formed on an outer periphery of the porcelain insulator. A seat member may be disposed between the shoulder and the piezoelectric sensor in direct contact with the shoulder for transmission of the pressure of combustion of fuel to the piezoelectric sensor through the seat member.
The housing may have an end portion which is far from the center electrode and staked on the porcelain insulator to retain the porcelain insulator within the housing. The seat member may alternatively be disposed between the staked end portion of the housing and the piezoelectric sensor in direct contact with the staked end portion for transmission of the pressure of combustion of fuel to the piezoelectric sensor through the staked end portion and the seat member.
According to the third aspect of the invention, there is provided a spark plug which comprises: (a) a center electrode; (b) a porcelain insulator having a first and a second length, the first length having the center electrode retained therein; (c) a hollow cylindrical housing in which the center electrode is retained through the porcelain insulator and which has a ground electrode installed thereon; (d) a piezoelectric sensor disposed on an outer periphery of the second length of the porcelain insulator, the piezoelectric sensor being responsive to pressure of combustion of fuel within an engine transmitted through the porcelain insulator to provide a signal indicative thereof; and (e) a holder staked on the housing so as to apply a given preload to the piezoelectric sensor at all times.
In the preferred mode of the invention, a chamber within which the piezoelectric sensor is disposed is filled with a resinous material.
The spark plug further comprises a shoulder formed on an outer periphery of the porcelain insulator and a seat member disposed between the shoulder and the piezoelectric sensor in direct contact with the shoulder for transmission of the pressure of combustion of fuel to the piezoelectric sensor through the seat member.
The housing may have an end portion which is far from the center electrode and staked on the porcelain insulator to retain the porcelain insulator within the housing. The seat member may alternatively be disposed between the staked end portion of the housing and the piezoelectric sensor in direct contact with the staked end portion for transmission of the pressure of combustion of fuel to the piezoelectric sensor through the staked end portion and the seat member.